In the present era of very large scale integration (VLSI) in which dimensions of transistor and other semiconductor structures are being reduced to less than one micrometer, isolation between devices has become very significant. One method of achieving the quality isolation needed for producing VLSI semiconductors is by using bonded wafer technology. By providing a thin layer of insulation between the semiconductor wafers, better isolation can be obtained for the devices formed in the semiconductor material, resulting in increasing speed, decreasing power dissipation, and improving transient radiation hardness of the semiconductor devices.
Although there are many variations and techniques for bonding wafers, the present methods have not been able to provide consistent and uniform results in obtaining a complete bonding of the wafers. For specific applications of bonding such as with silicon on insulator structures (SOI), this incomplete bonding can have significant adverse effects on the SOI devices produced from the bonding process.
The inventor has discovered that the prior art procedures used for bonding wafers are a significant cause of the inconsistency in bonding of the wafers. In the standard procedure, a pair of wafers are arranged contiguously together and placed upright in a standard quartz container, referred to as a boat. The boat and wafers are then placed in a furnace for about three to four hours at a temperature of approximately 1050.degree. C.
It is known that as the wafers are being heated, excess gases and water molecules are formed at the interface of the wafers. As these gases and water molecules accumulate at this interface, they will form voids in between the wafers unless the gases and water molecules are allowed to escape from between the wafers. The present procedures of bonding wafers in a standard boat does not allow for these excess gases and water molecules to effectively escape, thus inducing the voids to occur.
It is believed that the gases and water molecules are prevented from escaping because the standard boat container used in the bonding process provides several distinct heat sinking points along the circumference of the combined wafers. These heat sinking points will create intermittent cool spots on the circumference thereby further creating intermittent areas of nonheat sink locations. It is in these latter areas where the actual bonding of the wafers begins. It is the plurality of beginning bonding areas which inhibit the accumulated gases and water molecules from escaping from between the wafers. This results in trapping these gases and water molecules in between the wafers which leads to incomplete bonding. Because the number of beginning bonding areas varies due to the dependence of the placement of the wafers in the boat, the number of voids for each bonded wafer will also vary thereby providing the basis for inconsistent bonding results.